Triple multiplexing spread spectrum receiver

ABSTRACT

A spread spectrum receiver processes signals from a plurality of sources modulated by different spread spectrum codes by sampling the signals as received to produce an integer series of sampling segments at a sampling rate at least twice a chip rate of the codes, each sampling segment containing an integer number of bits representing a fraction of a chip of the codes, time division multiplexing each sample segment into a number of channels, correlating the bits in each sample segment in each channel in parallel with a source specific series of locally generated sequential code samples differing by one bit, summing each parallel correlation, and accumulating the summed parallel correlations for each code sample in each channel at a rate at least equal to the chip rate to derive data related to each of the sources.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.09/260,440, filed Mar. 2, 1999, which is a continuation of U.S.application Ser. No. 08/638,021, filed Apr. 25, 1996, now U.S. Pat. No.5,901,171, which claims the benefit of U.S. provisional application No.60/013,514, file Mar. 15, 1996.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates in general to spread spectrum receiversand in particular to GPS navigation systems such as those used interrestrial navigation for cars, trucks and other land vehicles.

[0004] 2. Description of the Related Art

[0005] Car navigation is conventionally performed using highway andstreet maps aided, to some degree, by distance measurements fromexternal sensors such as odometers. Improvements over the last 10 yearsin Global Positioning System, or GPS, satellite navigation receivers hasspawned several GPS car navigation systems.

[0006] Conventional GPS car navigation systems use the last knownposition of the vehicle, and the destination data, to compute a routedata base, including route and turning data derived from a pre-existingmap data base. GPS receivers are conventionally operated with a minimumof 3 or 4 satellites distributed across the visible sky in order todetermine, or at least estimate, the four necessary unknowns includingx_(user), y_(user) and z_(user) which provide three orthogonalcoordinates to locate the user as well as t_(user) which provides therequired satellite time. Techniques such as time or clock hold andaltitude hold, in which the unknown time or altitude is assumed toremain predictable from a previously determined value, e.g., z_(est)and/or t_(est), have permitted operation of GPS receivers with less than4 satellites in view. In particular, terrestrial GPS receivers have beenoperated with as few as 2 satellites to provide a 2 dimensional positionsolution using both clock and altitude hold.

[0007] Because continuous reception from 4 GPS satellites is oftendifficult to maintain in a car navigation environment, and known clockand altitude hold techniques can only permit operation with at least 2satellites, known conventional car navigation systems have typicallyaugmented the GPS position information with information from externalsensors to provide dead reckoning information. The dead reckoninginformation is often provided by an inertial navigation system such as agyroscope.

[0008] Augmenting GPS data with inertial navigation data has permittedthe use of GPS car navigation even when less than 4 satellites arevisible, such as in tunnels and in urban situations between tallbuildings. However, the resultant increased complexity and costs forsuch combined systems have limited their acceptance.

[0009] Conventional GPS receivers use separate tracking channels foreach satellite being tracked. Each tracking channel may be configuredfrom separate hardware components, or by time division multiplexing ofthe hardware of a single tracking channel, for use with a plurality ofsatellites. In each tracking channel, the received signals areseparately doppler shifted to compensate for the relative motion of eachsatellite and then correlated with a locally generated, satellitespecific code.

[0010] During a mode conventionally called satellite signal acquisition,delayed versions of the locally generated code for the satellite beingacquired are correlated with the doppler rotated received signals tosynchronize the locally generated code with the code, as received forthat satellite, by determining which delay most accurately correlateswith the code being received. Once synchronization has been achieved fora particular satellite, that satellite channel progresses to a trackingmode in which the doppler rotated, received signal is continuouslycorrelated with the locally generated code for that satellite todetermine position information including pseudorange information. Duringtracking, conventional receivers also correlate the doppler shiftedreceived signal with one or more versions of the locally generated codeat different relative delays, such as one half C/A code chip width earlyand late relative to the synchronized or prompt version of the code.These early and late correlations are used to accurately maintain thesynchronization of prompt correlation.

[0011] When, after tracking has begun for a particular satellite, thesatellite signal has been lost so that the required timing of thelocally generated code for synchronization is no longer accuratelyknown, conventional receivers reenter the acquisition mode, or a limitedversion of this mode, to reacquire the satellite signals by multiplecorrelations to resynchronize the locally generated code with the codeas received. Once the locally generated code has been resynchronizedwith the signals as received, position information data is again derivedfrom the signals from that satellite.

[0012] What is needed is an improved spread spectrum receiver, forexample, for use in a GPS system for terrestrial navigation which doesnot require the complexity or costs of conventional systems. Inparticular, a GPS receiver is needed which can make optimal use ofsignals available in difficult environments, such as urban environments,with less than 4 satellites continuously in view and frequentobscuration of signals from some of the satellites.

Summary of the Invention

[0013] In a first aspect, the present invention provides an improvedterrestrial navigation system using a GPS receiver which can continue tonavigate with continuous GPS data from less than the 3 or 4 GPSsatellites commonly required. The GPS data is augmented with data fromanother source. The source of the augmentation data may include datafrom external sensors, data bases including map data bases, and/orknowledge of the physical environment within which the vehicle is to benavigated. The use of such augmentation data permits GPS satellitenavigation solutions for stand-alone GPS systems as well as for GPSsystems integrated with external sensors and/or map databases with lessthan 3 or 4 continuously visible GPS satellites.

[0014] In another aspect, the present invention provides a GPS receiverin which map data used to determine routing is also used as a source ofdata augmentation for a single satellite solution by providing directionof travel information.

[0015] In still another aspect, the present invention provides a methodof augmenting GPS data using information from the physical environment.For example, vehicles are usually constrained to tracks no wider thanthe width of the roadway—and often to tracks only half the width of theroadway—and trains are constrained to the width of their tracks. Thiscross track constraint data may be used to provide augmentation data andallow the vehicle to continue to navigate with only a single satellitein view. The cross track constraint data permits the computation ofalong track data useful for calculating total distance traveled toprovide a GPS based odometer measurement.

[0016] The present invention permits the computation of distance alongtrack for use as an odometer reading while tracking only one satellite.Cross track hold provides along-track data directly which, in the caseof a vehicle, directly provides distance traveled information useful inlieu of a conventional odometer reading.

[0017] In addition to clock and altitude hold, the present inventionuses a technique which may be called cross-track hold in which thesingle satellite in view is used for determining the progress of avehicle such as a car along its predicted track, such as a roadway. Thedata conventionally required from a second satellite is orthogonal tothe track and therefor represents the appropriate width of the roadway.This value may be assumed and or constrained to a sufficiently smallvalue to permit an estimate of the value, e.g. y_(est) to provide a modedescribed herein as cross-track hold while obtaining useful GPSnavigation from a single satellite in view.

[0018] In other words, in accordance with the present invention, singlesatellite navigation may be achieved by using the data from the singlesatellite for on-track navigation information while holding orestimating the time, altitude and/or cross-track navigation data.

[0019] The required augmentation data may additionally, oralternatively, be derived from other sources in the physicalenvironment, such as turns made by the vehicle during on-track travel.In accordance with another aspect of the present invention, the vehiclemay detect turns made during travel and update the current position ofthe vehicle at the turn in accordance with the timing of the turn. Turndetection may be accomplished by monitoring changes in the vehiclevector velocity derived from changes in the GPS derived positioninformation or by monitoring changes in the compass heading or by anyother convenient means.

[0020] In another aspect, the present invention provides a GPS systemfor navigating a vehicle along a track, including means for tracking atleast one GPS satellite to provide on-track information related toprogress of the vehicle along a selected track, means for providing anestimate of cross track information related to motion of the vehicleperpendicular to the track, and means for providing vehicle navigationdata, such as vehicle position or vehicle velocity, from the on-trackinformation and the cross-track estimate.

[0021] In still another aspect, the present invention provides a methodof deriving position information from a single GPS satellite by trackingat least one GPS satellite to provide on-track information related toprogress of the vehicle along a selected track, providing an estimate ofcross track information related to motion of the vehicle perpendicularto the track, and determining the position of the vehicle from theon-track and the cross-track estimates.

[0022] In still another aspect, the present invention provides a methodof updating GPS position information for a vehicle navigating onroadways by deriving an indication that the vehicle has made a turn at aparticular point along a predetermined track, comparing the turnindication with stored navigation data to select data related to one ormore predicted turns at or near the particular point, comparing the turnindication with the predicted turn data to verify that the indicatedturn corresponds to the predicted turn, and updating GPS positioninformation to indicate that the vehicle was at the predicted turnlocation at a time corresponding to the turn indication.

[0023] In still another aspect, the present invention provides a GPSsystem for navigating a vehicle, the system including means for trackingat least one GPS satellite to provide on-track information related tothe direction of travel of the vehicle along a selected track, and meansfor deriving vehicle navigation data from changes in the direction oftravel of the vehicle along the selected track.

[0024] In a still further aspect, the present invention takes advantageof the typical improvement in satellite visibility possible in urbanroadway intersections by providing a fast satellite reacquisition schemewhich permits data from otherwise obscured satellites to aid in thenavigation solution even though visible only for a short time, forexample, as the vehicle crosses an intersection in an urban environmentin which tall buildings obscure the satellites from view except in theintersection.

[0025] In a further aspect, the present invention provides a spreadspectrum receiver having means for providing a plurality of versions ofa locally generated signal related to a spread spectrum signal to bereceived, means for combining at least two of the versions of thelocally generated signal with the spread spectrum signal to produce aproduct signal related to each of the at least two versions, means forevaluating the at least two product signals to adjust a parameter of thethird version of the local signal, means for combining the adjustedthird version of the local signal with the spread spectrum signal toproduce a data signal, means for determining a predicted value of theparameter when the spread spectrum signal becomes unavailable, means forcombining an additional plurality of versions of the locally generatedsignal related to the predicted value with received signals to produceadditional product signals related to each of the additional pluralityof versions of the locally generated signal, means for evaluating theadditional product signals to produce a reacquired data signal.

[0026] In another aspect, the present invention provides a method ofoperating a receiver for coded GPS signals from satellites bycorrelating early, prompt and late versions of a locally generated modelof the code with signals received from GPS satellites to adjust a delayof the prompt version to track a selected satellite, maintaining apredicted value of the delay when the selected satellite is unavailable,correlating a plurality of different early versions of the locallygenerated code with signals received from satellites to producecorrelation products, correlating a plurality of different late versionsof the locally generated code with signals received from satellites toproduce correlation products, and reacquiring the previous unavailableselected satellite by selecting the version producing the largestcorrelation product above a predetermined threshold as a new promptversion of the code to track the satellite.

[0027] In a still further aspect, the present invention provides aspread spectrum receiver for a spectrum spreading code having a fixednumber of bits repeated during a fixed length time period from aplurality of transmitters having a first time slicing level for slicingthe time period of the transmitted code into a number of time segmentsevenly divisible into the twice the number of samples, a secondmultiplexing level for dividing each time segment into a number ofchannels, each of the channels being used for tracking one of thetransmitters, and a third level dividing each of the channels in one ofthe segments into a number of code phase delay tests.

[0028] In another aspect, the present invention provides a receiver forprocessing signals from a plurality of sources, each modulated by adifferent spectrum spreading code repeating at a common fixed interval,including a sampler for deriving digitally filtered I and Q samples froma composite of spread spectrum signals received from the plurality ofsources, means for segregating samples of the signals being receivedduring each interval into a number of time segments, a time divisionmultiplexer for segregating different versions of the sequential samplesinto each of a number of channels, each channel representing one of theplurality of sources, a correlator for correlating the version of thesample in each channel with a series of sequentially delayed versions ofthe spectrum spreading code applied to the signals from the sourcerepresented by that channel, and an accumulator associated with each ofthe series of delays in each of the channels for processing the resultsof correlations performed during one or more intervals to deriveinformation related to the signals.

[0029] These and other features and advantages of this invention willbecome further apparent from the detailed description and accompanyingfigures that follow. In the figures and description, numerals indicatethe various features of the invention, like numerals referring to likefeatures throughout both the drawings and the description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is an overview illustration of the operation of a carnavigation system according to the present invention.

[0031]FIG. 2 is a block diagram of the GPS car navigation systemdepicted in FIG. 1, used for improved navigation during reducedsatellite visibility.

[0032]FIG. 3 is a schematic representation of a single satellite channelof a GPS receiver used for fast satellite reacquisition.

[0033]FIG. 4 is a schematic representation of a portion of the singlesatellite channel shown in FIG. 3 in which an additional plurality ofsets of delayed code samples are correlated to provide a finer gradationof correlation intervals.

[0034]FIG. 5 is a functional block diagram of a preferredimplementation, on an ASIC, of the satellite tracking channels andassociated processing components of the GPS car navigation system shownin FIG. 1.

[0035]FIG. 6 is a functional block diagram of the Doppler Block of theGPS car navigation system shown in FIG. 1.

[0036]FIG. 7 is a functional block diagram of the Coder Block of the GPScar navigation system shown in FIG. 1.

[0037]FIG. 8 is a functional block diagram of the Correlator Block ofthe GPS car navigation system shown in FIG. 1.

[0038]FIG. 9 is a function block diagram overview showing theinterconnections between the Doppler, Code, Correlator and other blocksof the system described in FIG. 5.

[0039]FIG. 10 is a block diagram of the operation of the system, shownin FIGS. 5 and 9, illustrating the data path of the present invention.

[0040]FIG. 11 is a series of exploded time segments illustrating theoperation of the data path of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0041]FIG. 1 is an overview illustration of the operation of a GPS carnavigation system according to the present invention. The GPS carnavigation system, described below in greater detail with respect toFIG. 2, is mounted in car 10 which is moving along the center of roadway12. NAVSTAR satellite 14, in the lower left quadrant of the figure, isin view of car 10. A simulated GPS circular overhead display, positionedapproximately over intersection 22 of roadway 12 and roadway 16indicates that satellite 14 is between 0° and 45° degrees of elevationabove the horizon as viewed from car 10.

[0042] For the purposes of illustration, satellite 18 is positionedoverhead between the elevation angles of 0° and 45° degrees. However,the line of sight between satellite 18 and car 10 is obscured bybuildings 20 so that satellite 18 is not in view of car 10 at theposition along roadway 12 as shown. Similarly, the line of sight betweensatellite 19 and car 10 is obscured by buildings 21. However, as will bediscussed below, when car 10 crosses intersection 22, the line of sightbetween satellite 19 and car 10, when the car is in position 11 withinintersection 22, may momentarily be clear.

[0043] Turning now to FIG. 2, GPS car navigation system 24 is a firstembodiment of a car navigation system according to the present inventionwhich may be installed in car 10 of FIG. 1. GPS car navigation system 24includes GPS car system module 26 which is provided with signalsreceived from satellites by GPS antenna 28, data related to the thencurrent—and expected future—physical environment of car 10 by forexample map data base 30 and data input from the operator of the car byfor example input device 32. GPS car system module 26 provides output tothe operator, for example, in the form of a GPS map display, via displayunit 34, which may include both visual display as well as a voiceinterface announcing information as required to supplement or evenpartially replace visually presented data.

[0044] The present invention may be configured for use with only a GPSreceiver, a GPS receiver aided by map data from, for example, map database 30, and/or a GPS receiver aided by both a map data base as well asan external source of information, for example, from an external sensor.This external source of information may be used for maintaining positioninformation by dead reckoning during those times when a sufficientnumber of satellites are not in view to provide the desired information.

[0045] In operation, a composite of all signals received from NAVSTARsatellites is applied by GPS antenna 28 to satellite receiver section 36of GPS car system module 26. Signals from individual NAVSTAR satellitesare then tracked in satellite specific tracking channels such as SatTRAKchannels 38, 40, 42 and 44. Although it is quite conventional to track 4to 12 satellites and therefore use 1 to 12 satellite tracking channels,only 4 such channels are shown herein for clarity. The outputs of thesesatellite specific tracking channels are processed by SatProcessor 46 toprovide x_(user), y_(user), z_(user) and t_(user) data via appropriatelogic control to a GPS position processor, such as PosProcessor or NavSoln 48 which determines the navigation solution to determine positiondata. Position data is then applied by PosProcessor 48 to an appropriatedisplay for the operator of the car, such as display unit 34.

[0046] External sensor 49, in FIG. 2, may conveniently provide sensordata, or local or satellite position information, or positioninformation which provided local position or satellite positioninformation directly to PosProcessor 48 for comparison with the positioninformation determined by SatProcessor 46 and/or Map/Display Processor50. External sensor 49 may conveniently be any sensor which providesinformation useful for updating position information for dead reckoningincluding direction, speed, velocity or acceleration or other data fromwhich dead reckoning data may be derived. Conventional sensors includeinertial navigation systems, with magnetic or optical gyroscopes,fluxgate compasses, odometer or wheel sensors or the like.Alternatively, external GPS format signals, such as those provided by apseudolite, may be used to update current satellite or positioninformation.

[0047] At the beginning of a navigated trip, the operator of car 10would typically provide data concerning the physical environmentsurrounding the intended route to GPS car system module 26 by insertingan appropriate data storage device such as a CD ROM, into map data base30, and/or by entering the data via input device 32 which convenientlymay be a keypad, keyboard, pointing device, track ball, touch screen,graphical pad, a voice recognition interface and/or a combination ofsuch input devices. The operator of car 10 would also enter the intendeddestination into GPS car system module 26 via a data entry device suchas a mouse or track ball interacting with display unit 34 and/or viainput device 32. Map/Display Processor 50 of GPS car system module 26would then develop the desired route, typically from the then currentlocation as a point of origin to the desired destination, in accordancewith the rules of navigation and details of the locale provided by mapdata base 30. The appropriate route data is stored in Route Data Base52, including the routing in the form of roadways and turns betweenroadways. Additional information, such as altitude, width of theroadways and etc. may also be contained within map data base 30 and/orRoute Data Base 52. These data bases may be contained within GPS carnavigation system 24 and/or made available to GPS car system module 26from outside storage media such as diskettes positioned in appropriatedisk drives.

[0048] During navigation, each satellite in view may be tracked in asatellite tracking channel. If, for example, 4 or more satellites are inview, each of the satellites in view will be tracked in an individualchannel, such as SatTRAK channels 38, 40, 42 and 44. The output of thesatellite tracking channels is then applied to SatProcessor 46 whichwould provide satellite based solutions of the four unknowns, such asx_(user), y_(user), z_(user) and t_(user). The data represented byx_(user) and y_(user) are conventionally used as the two dimensionalorthogonal components of the surface of the earth such as north andeast. However, in accordance with the present invention, x_(user) andy_(user) are preferably used to represent data for a pair of orthogonaldirections specific to the direction of vehicle travel called theon-track and cross track directions.

[0049] Conventional bearing, such as north, south, east and west arerelative to the magnetic or true north poles of the earth, whileon-track and cross-track, as used in the present invention, are bearingsmade relative to the expected direction of travel of car 10 at anyparticular point in the route. For example, while a 90° turn from aheading of due north would change the angle of the vehicle velocityvector from 0° to 90° if bearings relative to the Earth's surface suchas north and east are used, the same turn would show no change in the 0°angle of the vehicle velocity vector before or after the turn as long ascar 10 remained on the expected track.

[0050] The data represented by z_(user) is typically surface elevation,such as the elevation above sea level, while the data represented byt_(user) is the exact time as determined from one or more of thesatellite tracking channels.

[0051] Solutions for all 4 unknowns of position information may bederived from signals from 4 satellites in view, so that exact positioninformation within the limit of the accuracy then available from the GPSsatellite constellation in view can therefore be applied by PosProcessor48 to Map/Display Processor 50. The position information determined fromthe satellites is processed with the physical data from map data base30, and/or the desired routing data from Route Data Base 52, to provideappropriate navigation information to the operator of car 10 via displayunit 34.

[0052] If less than 4 satellites are in view, the t_(user) solutionapplied to PosProcessor 48 may be replaced by t_(est) 54 estimatedsolution derived for example from an internal clock model 54 in positionestimate or model 63. Similarly, the z_(user) solution may be replacedby z_(est) 56 solution derived from elevation estimate 56, also inposition model 63, in accordance with routing data derived from RouteData Base 52 in accordance with then current GPS position informationapplied to Map/Display Processor 50. T_(est) 54 and z_(est) 56 areapplied to PosProcessor 48, and used in lieu of t_(user) and z_(user),when only two satellites are in view. The use of estimated or modeledsolutions for the t and z variables, that is the use of t_(est) 54 andz_(est) 56 are conventionally known as clock hold and altitude hold,respectively.

[0053] It must be noted that the particular configuration of GPS carnavigation system 24 as described so far is only one of the many knownways of configuring such systems any of which may be used withoutdeparting from the spirit or scope of the present invention as definedby the claims.

[0054] In accordance with the present invention, the width of theroadway, either known or estimated, may be used to provide y_(est) 60for use in lieu of y_(user) when only one satellite is visible. y_(est)60 may be derived from Route Data Base 52 and/or map data base 30. Sincethe x and y unknowns are orthogonal, x_(user) may be used to describethe on-track information, that is, the progress of car 10 along itspredetermined track while y_(est) 60 represents the cross trackinformation, that is, how far car 10 has strayed from the center of theroadway.

[0055] Referring therefore to FIG. 1, x_(user) is used to indicate theprogress of car 10 along roadway 12 while y_(est) 60 is used torepresent the width of roadway 12. The actual width of the roadway maybe derived from map data base 30, or assumed because the actual value ofthe width of the roadway is relatively small and often thereforeinsignificant compared to the distances to be measured along thenavigation route. Since the maximum allowable cross-track error, i.e.the maximum allowable appropriate value for y, is constrained by thephysical width of the roadway, y_(est) 60 is relatively easy toaccurately estimate.

[0056] By using y_(est) 60, z_(est) 56 and t_(est) 54, it is possible toprovide useful navigation data for car 10 along a known roadway usingsignals from only a single satellite in view. It is important to notethat reasonably accurate prior or initial position information may berequired and that not all visible NAVSTAR satellites will be suitablefor single satellite navigation, depending upon the position of thesatellite with respect to the path of car 10. The position informationdetermined during single satellite navigation is along track positioninformation which may be accumulated and used for determiningaccumulated along track distance traveled. This data provides, and maybe used in place of, the distance traveled information conventionallyprovided in a vehicle by an odometer.

[0057] Referring now to both FIGS. 1 and 2, turning data may be used toimprove terrestrial GPS navigation by using the detection of a knownturn to update progress along a predetermined route. When at least 4satellites are in view, the position of car 10 may be known to theaccuracy of the GPS system. When using clock, altitude or cross-trackhold, or some combination thereof, the known position of the car isdegraded by any inaccuracy of the estimate or estimates used. Forexample, during clock hold, internal clock model 54 drift and inaccuracyof the source of t_(est) 54 will degrade the accuracy to which theposition is known as a function of the magnitude of the inaccuracy.Similarly, any change in altitude from the estimated or fixed altitude,that is, any inaccuracy of z_(est) 56, will degrade the accuracy of theknown position. Changes in roadway width and inaccuracies in the mapdata with regard to the roadway width, that is, any inaccuracy iny_(est) 60, may also degrade the position information.

[0058] Even with 4 satellites in view, the geometry of the visiblesatellites may make it difficult to determine position by measurement ofGPS signals. Further, during terrestrial navigation, it is not uncommonfor satellites to be temporarily obscured from view during navigationby, for example, being blocked by buildings and other obstructions.

[0059] It may therefor be desirable to update the accuracy with whichthe current position of the vehicle is known with actual positioninformation whenever possible. The update information will sometimes beuseful when 4 satellites are in view, but will always be useful assupplemental data when less than 4 satellites are in view. Updateinformation is extremely useful during single satellite navigation toavoid the accumulation of errors in position information.

[0060] In operation, an original position and a destination wereprovided to the system which then determined the track to be followed.The track to be followed, or routing information, may be provided in theform of a data base of route information such as Route Data Base 52. Inthe example used, track 62 follows the centerline of roadway 12 tointersection 22 and then makes turn 64 to follow the centerline ofroadway 16. Track 62, roadways 12 and 16, intersection 22 and turn 64are provided to Route Data Base 52 during the preparation of the routeby Map/Display Processor 50 from the then current position and thedestination entered via input device 32.

[0061] The physical position of car 10 is very accurately known when car10 turns at turn 64. This accurate knowledge of the car's position at aparticular time may conveniently be used to update the GPS navigationinformation by providing a position reset which is similar to a knowninitial position. Update information from turns will most likely beuseful if the angle of turn 64 is sufficiently large to provide anunambiguous position determination. It is expected that any turn greaterthan 45° may be detected. As the speed of the vehicle increases, smallerturn angles may also provide useful information. The position updateinformation is applied to position model 63 to update internal clock ort_(est) model 54, elevation or z_(est) model 56, y_(est) model 60 aswell as x_(est) 61 which is a model of the along-track position of thecar. These four estimates together for position model 63, which may beupdated by information from map data base 30, Route Data Base 52,current position processor 70, PosProcessor 48 and/or external sensor49, to form the most accurate available position model 63. Positionmodel 63 may also be used to provide estimates to the same data sources.

[0062] The actual turning of the car may be detected by a change in thevehicle velocity vector determined from the GPS data or from otherconventional means such as a magnetic compass or an inertial navigationsensor. In accordance with the rapid reacquisition system describedbelow with respect to FIG. 3, GPS data alone may conveniently detectsuch turns even when single satellite navigation is required. The turnas detected by turn detector 66 is correlated with data from Route DataBase 52 to determine the actual position of the car to the accuracy ofthe map data base 30. The accuracy of the data in map data base 30 mayeasily and conveniently be much greater than the accuracy available fromthe GPS system especially if single satellite navigation, or anycombination of clock, altitude or cross-track hold, is used. Thereforethe position update may provide a substantial increase in the accuracyof the then current position determination.

[0063] The benefit of the approach of this embodiment of the presentinvention is similar to the identification and use of a known waypointduring a dead reckoning navigation run. The cumulative error is reducedsubstantially at the known waypoint so that additional, future positiondetermination errors do not carry the burden of an accumulation of pasterrors.

[0064] As shown in FIG. 2, Route Data Base 52 provides data related totrack 62, typically from Map Data Base 30, to Map/Display Processor 50to display the current GPS position and may also provide similarinformation to turn detector 66, turn comparator 68 and/or currentposition processor 70 in order to update PosProcessor 48 with a positionreset.

[0065] Turn detector 66 may be configured in many different ways and isused to detect turns actually made by car 10 and select turns, such asturn 64, from Route Data Base 52 for later comparison with the detectedturn. In accordance with a preferred embodiment of the presentinvention, turn detector 66 may operate on the current GPS positionprovided by PosProcessor 48 to develop a vehicle velocity vectorposition indicating both the direction and speed of travel. Substantialchanges in the direction portion of the vehicle velocity vector wouldindicate a change in direction, such as a turn. Turn detector 66 maytherefore detect turns directly from the GPS information by determiningthe vehicle velocity vector and detecting changes in the vehiclevelocity vector which represent a turn.

[0066] Turn detector 66, or another unit if convenient, also operates onthe route information provided by Route Data Base 52 to determine theexpected position of car 10 along track 62 based on the then current GPSposition information. Once the expected location of car 10 along theroute is determined, one or more turns in the area of the expectedposition of car 10 can be selected for comparison with the indicationsof a physical turn derived from the GPS data.

[0067] When changes in the actual vehicle velocity vector, as derivedfor example from the GPS position data, compare appropriately with thechanges predicted at a particular turn as derived from Route Data Base52, the actual position of car 10 at the time of the turn can be veryaccurately determined and used to update the GPS data at the turn. Forexample, if an actual turn is detected from a change in the vehiclevelocity vector from the GPS position of car 10 near the time predictedfor that turn, the actual position of car 10 at the time of the turn canbe determined and used to update the then current GPS position for useas a position reset applied to PosProcessor 48.

[0068] Alternatively, turn detector 66 may use non GPS measurements fordetermining the occurrence of an actual turn of car 10, such as compassheadings or inertial navigation determinations derived for example fromexternal sensor 49, and applied directly to turn detector 66 or viaPosProcessor 48 as shown in FIG. 2.

[0069] Detection of turns from GPS signals may easily be accomplished aslong as 2 satellites are in view and provide appropriate geometries fordetermining two dimensional coordinates of the car's position. Duringsingle satellite navigation, as described above, the use of turninformation for updating the last known position information becomeseven more important, but the location of the single satellite in view,relative to track 62, becomes of even greater importance so that actualturns may be accurately detected.

[0070] Turn detection may also be provided by monitoring changes betweenacquired and obscured satellites. If, for example, only satellite 14 wasvisible to car 10 on roadway 12 before intersection 22, and uponentering intersection 22, satellite 19 suddenly became visible whilesatellite 14 was momentarily obscured, the change over from satellite 14to satellite 19 could be used to indicate a turn in accordance with thedata from each satellite. Using a rapid reacquisition scheme, asdescribed herein below, the actual position at which the change ofdirection, that is, the switch between satellites, can be sufficientlyaccurately determined to permit precise position update information atthe turn.

[0071] Similarly, turn comparator 68 may conveniently be implementedwithin another component of the system, such as PosProcessor 48,Map/Display Processor 50 and/or SatProcessor 46, so that a candidateturn may be selected from the route data for track 62 for comparisonwith the detected turn data.

[0072] Referring now to FIG. 3, in another embodiment, the presentinvention provides for fast reacquisition of satellite signals, usefulfor example when a previously acquired satellite is obscured and thenappears perhaps for only a short time, for example, as a car travelsthrough an intersection.

[0073] Referring to the line of sight between car 10 and satellite 19 asshown in FIG. 1, it is common in an urban environment for the buildingsalong the sides of the street to act as a barrier wall obscuring thelines of sight to many GPS satellites. However, the barrier wall formedby buildings 20 and 21 is commonly breached at intersections such asintersection 22. For example, car 10 while traversing intersection 22may reach position 11 in which the previously obscured line of sight toa satellite, such as satellite 19, is momentarily not obscured becauseof the break between buildings 20 and 21 at intersection 22. Thismomentary visibility of a previously obscured satellite may occur whilecar 10 is in the intersection or at the edges of the intersection.

[0074] The length of the momentary contact with satellite 19 isrelatively short. For example, if intersection 22 is 60 feet wide andcar 10 is traveling at 30 mph, the time taken to cross the intersectionmay be as short as 1.3 seconds. Conventional GPS navigation systemswould not reacquire and derive useful data from satellite 19, even ifpreviously acquired, during this short time interval.

[0075] In accordance with another embodiment, the present inventionmakes maximum use of such reacquisition opportunities by minimizing thetime required for reacquisition, the collection of data and processingof the collected data for position determination. Referring now to FIG.3, a portion of SatTRAK channel 38 is shown in greater detail as anexample of the configuration of each of the satellite tracking channels.After original acquisition, SatTRAK channel 38 tracks a single satelliteby operating on satellite signals 72 received by GPS antenna 28.Satellite signals 72 include the signals from the satellite beingtracked by SatTRAK channel 38 and are demodulated and selected by beingmultiplied, in one of the correlators 74, by a copy of the 1023 chippseudorandom, spread spectrum code applied to satellite signals 72 bythe GPS satellite. Correlators 74 may be configured from exclusive OR orNOR gates to minimize the time required for providing a correlationresult.

[0076] During tracking, the copy of the code produced by code generator76 and applied to exclusive OR correlators 74 by delay 78 issynchronized with the code in satellite signals 72, as received, so thatthe copy of the code correlates with satellite signals 72. This may beaccomplished in several different manners known in the art, including byshifting the time of generation of the code in code generator 76 and/oradjusting the amount of delay applied by an external delay. In anyevent, the code applied to exclusive OR correlators 74 when SatTRAKchannel 38 is locked to the selected satellite, is synchronized with thecode being received from that selected satellite. This correlation iscommonly called the on-time or prompt correlation to indicate thissynchronization.

[0077] Conventional GPS receivers maintain a lock on a satellite signalafter acquisition by performing additional correlations, often calledearly and late correlations or correlations performed by early and latecorrelators. These correlations are displaced in time by a certain delaysuch as one half the width of a C/A code chip from the on-time or promptcorrelator. That is, if the time of occurrence of a particular chip inthe satellite signals is time t0, the prompt correlator under idealconditions would multiply satellite signals 72 with a replica of thecode with the same chip at time t0. The early correlation would beperformed at time t0 −½ chip and the late correlation would be performedat a time equal to t0 +½ chip. Whenever the synchronization between codegenerator 76 and satellite signals 72 as received begins to drift, thecorrelation results begin to change in favor of either the early or latecorrelation at the expense of the prompt correlation.

[0078] One conventional approach to maintaining lock on the signals froma particular satellite is to adjust the timing of code generator 76 witha feed back loop used to maintain the power in the correlation productsin the early and late correlators to be equal. In this way, codegenerator 76 may be continuously resynchronized with satellite signals72 so that the accuracy of the system is within one half chip in eitherdirection (early or late) of the signals received.

[0079] When satellite signals 72 are temporarily lost, for example,because the satellite signals are temporarily obscured by buildings 20and 21 as shown in FIG. 1, various techniques are used to attempt tosynchronize code generator 76 with satellite signals 72 as received sothat SatTRAK channel 38 can reacquire the signals from the desiredsatellite. As noted above, conventional techniques include clock andaltitude hold and one embodiment of the present invention providesanother technique called cross-track hold.

[0080] However, unless the obscuration of the satellite signals is verybrief, the accuracy of prediction of such techniques is not enough tomaintain synchronization except for a very brief period of obscuration.

[0081] In accordance with another embodiment of the present invention,massively parallel correlation is used to create an expanded capturewindow of correlation capture around the then current predictedsynchronization time in order to immediately reacquire a previouslyacquired, and then obscured, satellite signal. In particular, the speedof reacquisition is made sufficiently fast according to the presentinvention so that useful GPS position data may be acquired during thetime car 10 travels through intersection 22 even though, for example,the signals from satellite 19 were obscured by buildings 20 until car 10was within intersection 22.

[0082] To this end, an expanded series of correlations are performedwith a series of delays a fixed fraction of a chip width, such as ½ chipwidth, apart extending both early and late of the predicted promptcorrelation. As shown in FIG. 3, satellite signals 72 are devolved intoa fixed number of samples, by for example analog to digital conversionin A/D Converter 73, to provide n Signal Samples 75. A similar number ofcode samples are provided through k fixed ½ chip width delays 78 toprovide k-1 sets of n Code Samples 80, progressing from a first set of nCode Samples 80 with no delay to the k-1st set of n Code Samples 80which have been delayed by a total of k delays 78. It is convenient touse ½ chip delays for each delay 78, but other fractions of a chip widthmay be used.

[0083] The k/2th set of n Code Samples 80, or the set nearest k/2, mayconveniently be delayed the correct amount to perform the promptcorrelation in one of the exclusive OR correlators 74 with n SignalSamples 75 from A/D Converter 73 during tracking. The k/2th−1 set of nCode Samples 80 may then be used to perform the early correlation whilethe k/2th+1 set of n Code Samples 80 may then be used to perform thelate correlation while tracking. additional correlations may also beperformed during tracking, but provide a substantial advantage when usedduring reacquisition.

[0084] That is, in the present invention, the early, prompt and latecorrelations conventionally used in tracking may also be used duringreacquisition mode, aided by a substantial number of correlations usingadditional delays. Whether or not the early and late correlations areused, a convenient number of additional delays on each side of theprompt delay results from (k-1)=20 so that nine or ten ½ chip delays areprovided on each side of the k/2th prompt delay. In this way,correlations are performed during reacquisition at time delays of 5 chipwidths on either side of the predicted prompt or on-time delay. Thisrepresents an expanded capture window of on the order of ±5×300 metersof potential error. That is, if the predicted synchronization withsatellite signals 72 modeled by GPS car system module 26 drifted by asmuch as the equivalent of a ±1500 meter position error during signalloss from a particular satellite resulting from, for example obscurationin an urban setting, at least one of the plurality of exclusive ORcorrelators 74 would provide the required prompt correlation toimmediately lock onto satellite signals 72.

[0085] Once the correlations are performed, the correlation results foreach set of n Code Samples 80 are summed in summers 84 to produce aseries of values each separately indicating the correlation of n SignalSamples 75 with each of the sets of n Code Samples 80. These correlationresults are applied to threshold test 82, the output of which is appliedto SatProcessor 46 only when satellite signals 72 have been successfullyreceived. The output of threshold test 82 specifies the number of delayswhich represent the prompt correlation for the reacquired satellitesignal. It is important to note that in accordance with the presentinvention, the satellite tracking and reacquisition modes are notseparated functions but rather interact seamlessly. That is, byproviding a substantially expanded capture window, the correlations usedfor tracking are also automatically useful for immediate reacquisitionas long the capture window is sufficiently wide to include any positionerror accumulated during signal obscuration or other loss.

[0086] Because the speed of reacquisition is very important in order tomaximize the opportunity to utilize the brief time during travel throughintersection 22 when satellite 19 may temporarily be in view, it isadvantageous to perform all such correlations in parallel. Further, itis advantageous to continuously perform all such correlations in thecapture window in order to minimize time when a satellite signal is notbeing tracked. In accordance with the presently preferred embodiment,exclusive OR correlators 74 are implemented in hardware rather thansoftware to maximize the speed of correlation and minimize any erroraccumulation by minimizing the time for reacquisition.

[0087] In operation, when car 10 follows track 62 along roadway 12,during at least part of the time buildings 21 obscure the line of sightbetween car 10 and satellite 19. If satellite 19 had previously beenacquired by GPS car system module 26, an approximate time value tosynchronize with the satellite signals will be predicted. This value ismaintained as accurately as possible within GPS car system module 26while satellite 19 is obscured. In order to maintain the prediction forthe required delay as accurately as possible, that is, to minimize theposition error accumulated during signal loss, the above describedtechniques for maintaining or updating position accuracy by usingcross-track hold, resetting position at a determined turn and/or the useof external sensors for dead reckoning provide a substantial benefit foruse with the combined, expanded tracking and reacquisition windowsdescribed above.

[0088] Present technology makes it convenient to provide ½ chip delaysbetween correlators, but other delay values may be used. Similarly, itis convenient to expect that the prompt correlation can be maintainedwithin plus or minus 5 chips of the timing of the satellite signals.FIG. 3 therefore portrays a series of 9 or 10 early and 9 or 10 latecorrelators surrounding prompt correlator 74 to achieve the ±5 chipcapture window surrounding prompt correlator 74 in 20 half chip steps. Adifferent number of correlators and other delays would also work withthe present invention.

[0089] Use of a plurality of fixed delays of one half chip width permitthe immediate reacquisition of signals from a satellite to within anaccuracy of one half chip width. In accordance with satellite signals 72as presently provided by the NAVSTAR satellites, one half chip widthrepresents about 150 meters of maximum position error. It is possible tosubstantially reduce the maximum position error, and/or the speed ofprocessing the data, by using fixed delays of a different amount ofdelay, e.g. fixed delays of one third, one quarter, one fifth or someother value of a chip width.

[0090] Conventional approaches for different modes of operation, switchbetween wide and narrow delays at acquisition +/or reacquisition inorder to provide a compromise between the width of the capture windowand the number of correlations required for the desired range. Inaccordance with the present invention, a new technique is used whichpermits the convenient use of fixed, chip width delays to provide afiner gradation of correlation steps. In particular, as shown in FIG. 4,two sets of half width delays are used to provide the equivalent of aset of quarter width delays. The number of sets of fixed delays and theoffset between them may be selected in accordance with the requirementsof the application being addressed.

[0091] Referring now to FIG. 4, a first plurality of sets of n CodeSamples 80 are derived directly from code generator 76, delayed fromeach other by ½ chip width delays 78 and correlated with n SignalSamples 75 in exclusive OR (or NOR) correlators 74 as provided in FIG.3. For convenience of explanation and drawing, the outputs from thisfirst set of set of n Code Samples 80 are shown applied to summers 84 toindicate that the correlation products produced in exclusive ORcorrelators 74 from each such set of n Code Samples 80 are applied tothreshold test 82 via summers 84. All such correlation products areapplied, but for clarity only the correlation products having no delay,the predicted prompt or k/2th delay and the kth delay are depicted. Thecorrelation products from this first plurality of sets of n Code Samples80 are spaced apart by ½ chip width delay as noted above.

[0092] In addition, in accordance with the present invention, additionalsets of correlation products at different spacings are available by useof one or more additional sets of ½ chip delays 78 by, for example,tracking the same satellite in two or more channels offset in time fromeach other. It is important to note again that other delays and/oroffsets may also conveniently be used and the delays need not all be thesame.

[0093] In particular, a second plurality of sets of n Code Samples 84are derived from code generator 76 and delayed from each other by ½ chipwidth delays 78. However, the delays in the second sets of n CodeSamples 84 are offset from the delays in the first sets of n CodeSamples 80 by a fixed amount, such as a ¼ chip width delay, by insertionof ¼ chip width delay 79 between code generator 76 and the first set ofn code samples in sets of n Code Samples 84. This results in each of thesamples in sets of n Code Samples 84 falling halfway between two of thesets of n Code Samples 80. As shown in FIG. 4 only k−1 sets of n CodeSamples 84 are required with k sets of n Code Samples 80.

[0094] Each of the sets of n Code Samples 84 are correlated with nSignal Samples 75 in exclusive OR correlators 74 as provided in FIG. 3to produce correlation products which are then summed by additionalsummers 84. As noted above, the dashed lines between each of the sets ofcode samples and summers 84 are used to indicate that the correlationproduct between that set of code samples and n Signal Samples 75 isapplied to a particular one of summers 84. As can then easily beunderstood from FIG. 4, correlation products separated from each otherby ¼ chip width delays, from the 0th delay to kth delay, are producedusing sets of ½ chip width delays and a single ¼ chip delay (which mayrepresent the offset delay between two channels) and after individualsummation are applied to threshold test 82 to determine which delayrepresents the currently prompt delay of satellite signals 72 from asatellite being reacquired by GPS car system module 26.

[0095] The second set of ½ chip delays may easily be implemented byhaving a second channel track the same satellite, offset, however by ¼chip width delay 79.

[0096] In this way, the range of delay within which a satellite signallock may be acquired, maintained and/or reacquired may be reduced from±½ chip width, to about ±¼ chip width, which permits faster pull in tolock, i.e. when the tracking has been optimized and range error reducedto minimum.

[0097] It is important to note the seamless integration of tracking andreacquisition provided by the present invention in that the samecorrelations are used for tracking and reacquisition and the relatedspeed of capture and lock and simplicity provided thereby. The abilityto rapidly reacquire within a capture window so that one of thecorrelations may immediately be used as a prompt correlation, speeds upall data acquisitions thereafter.

[0098] It is also convenient to utilize a first plurality of sets of nCode Samples 80 for tracking and, when satellite signals 72 are lost,provide additional accuracy in reacquisition by using a second pluralityof sets of n Code Samples such as sets of n Code Samples 84. Inparticular, the same plurality of sets of n Code Samples 84 may be usedfor reacquisition of signals 72 for different satellites at differenttimes in order to reduce the total number of components and stepsrequired to produce all the necessary correlations and summations.

[0099] In operation, GPS car system module 26 continuously attempts totrack and reacquire the signals from satellite 19 in SatTRAK channel 38while satellite 19 is obscured from view. As car 10 passes throughintersection 22, the line of sight to satellite 19 is momentarily notobscured by buildings 21. Whenever any of the correlations performed inSatTRAK channel 38 indicate that the satellite signals are beingreceived with sufficient strength so that the correlation products fromsome of the correlators are above threshold, reacquisition isimmediately accomplished. Reacquisition occurs when the correlatoroutput indicating the largest magnitude is selected as the new promptcorrelator. Conventional techniques for improving the quality of thedata are then employed.

[0100] The data from satellite 19 is used to immediately, after asettling time for lock, update the GPS data and correct the currentlyknown position information derived therefrom. Even if satellite 19 isthen again immediately obscured, the update information derived duringtravel through the intersection by fast reacquisition provides asubstantial improvement in accuracy of the GPS determined position. Thiswill permit GPS car system module 26 to continue accurate navigationeven through otherwise very difficult areas, such as city streets.

[0101] Although the use of single satellite navigation data bycross-track hold and the updating a satellite data by detecting turnsand/or immediately reacquiring satellite signals in intersections haveall been described separately, they are also very useful in combination.Terrestrial navigation systems, using GPS receivers in a stand alonemode, aided by map displays and data bases and/or aided by externalsensors such as inertial navigation systems may benefit from the use ofcombinations of one or more such modes. In a preferred embodiment of thepresent invention, all three techniques are combined to maximize theability of the car navigation system to provide accurate and usefulnavigation data while traversing a difficult environment such as citystreets.

[0102] Referring now to FIG. 5, a preferred embodiment of the presentinvention is described in which major portions of SatTRAK channels 38,40, 42 and 44 and SatProcessor 46 of the present invention areimplemented in an Application Specific Integrated Circuit or ASIC 102.Many of the functions of a conventional satellite processor may still,however, be performed in software. The particular implementationdepicted provides a 12 channel GPS acquisition and tracking system withfast reacquisition capabilities as described above while substantiallyreducing the number of gates required on the ASIC to implement thissystem.

[0103] The signals received by GPS antenna 28 are digitized and form adigital composite of signals received from all satellites in view toproduce sample data 100 which is at a frequency of 37.33 f₀ where f₀ isthe chip rate of the C/A code applied to each GPS satellite. Forconvenience, the frequencies described below will be designated in termsof multiples of f₀. Each of 12 Space Vehicles (SVs) or satellites aretracked in ASIC 102 under the control of Central Processing Unit, orCPU, 101 which provides control signals and data to ASIC 102. Inparticular, CPU 101 provides data regarding the predicted Doppler shiftsand C/A code applied to each SV to Random Access Memory, or RAM, R1 103associated with ASIC 102 which provides the data to RAM R2 105 atdesignated times. RAM R2 105 provides data to and receives data fromASIC 102, permitting CPU 101 data updating and ASIC 102 processing ofold data to operate simultaneously. RAM R2 105 is used by ASIC 102primarily to store intermediate values of signals during processing.Other conventional portions of a micro-computer including a CPU are notshown but conveniently may include devices operating softwareimplementing the single satellite, cross-track hold and other techniquesdescribed above as well as other functions of SatProcessor 46.

[0104] Sample data 100 is applied to C/A code acquisition, tracking andreacquisition block CACAPT 104 in ASIC 102 where it is split intoin-phase and quadrature-phase, or I and Q, signals at baseband by I/Qsplitter 106. After processing by CACAPT 104, the I,Q signals arerotated for Doppler shift in 12 channel Doppler Block 108 whichseparately compensates for the expected Doppler frequency shifts of eachof the 12 SV's which can be tracked.

[0105] The Doppler rotated I,Q signals for each SV are then applied toCorrelator Block 110 where each signal sample, which is from one of the12 SVs, is correlated in a multiplexed fashion with 20 delayed versionsof the C/A code, produced by 12 channel Coder Block 112, for that SV.During each segment of time, as described below with regard to FIG. 11in greater detail, Correlator Block 110 performs 240 C/A codecorrelations in accumulator 175 to enhance the speed of acquisition andreacquisition. The output of Correlator Block 110 is applied to IQACCUMBlock 114, and the output of IQACCUM Block 114 is applied to IQSQACCUM116, in Accumulator Block 115. IQACCUM Block 114 is convenientlyconfigured from another block of RAM associated with ASIC 102,identified herein as RAM 3. Similarly, IQSQACCUM 116 is convenientlyconfigured from another block of RAM associated with ASIC 102,identified herein as RAM 4.

[0106] Accumulator Block 115 operates in different fashions duringacquisition, tracking and reacquisition modes under the direction of CPU101. During acquisition mode, Coder Block 112 is caused to sequencethrough as many sets of 240 different codes delays as necessary toacquire the satellite signals from a particular space vehicle. That is,as many sets of 240 different delays are correlated in Correlator Block110 to provide IQSQACCUM 116 with an appropriate correlation outputwhose power indicates that correlation has been achieved with thatsatellite. The process is then repeated for each satellite to beacquired. For convenience, all delays may be tested.

[0107] During reacquisition, a single set of 20 delays are correlated inCorrelator Block 110 to determine if one such delay provides a peakvalue above a predetermined threshold to indicate that a correlation hasbeen achieved and the satellite thereby reacquired. The reacquisitionmode operates transparently within the tracking mode in that a set of 20delays are correlated in Correlator Block 110. If tracking ismaintained, the peak signal may migrate from a particular delay to thenext adjacent delay but will be maintained within the current set of 20delays being correlated. It is convenient to consider the delayproducing the signal with the greatest magnitude as the promptcorrelation product. The signals produced by one more and one less delaythen become the early and late correlation products which may beprocessed in a conventional manner to maintain lock with each satellite.

[0108] If the signal from the satellite is temporarily obscured or lostfor any other reason, the then current set of 20 delays is correlated tosearch for a peak of sufficient magnitude to indicate reacquisition. Thedoppler and code values are continuously updated based upon the lastavailable position information including velocity, and the correlationsare performed, until the satellite signal is reacquired or sufficienttime has elapsed so that the satellite signal is considered lost.

[0109] The operation and configuration of ASIC 102 will now be describedin greater detail with regard to the in-phase or I signal path. Thequadrature phase or Q signal path is identical and need not beseparately described.

[0110] Within CACAPT 104, sample data 100 is applied at 37.33f₀ to I/Qsplitter 106 to produce a 2 bit signal at 18.67f₀which is furtherreduced to 2f₀ by Digital Filter 118 which operates by adding sets of10, 9 and 9 samples which are summed, quantized, and then storedserially in 11 sample deep buffer 120. When 11 sample deep buffer 120 isfilled, the data is transferred in a parallel fashion to an identicalbuffer, called parallel block 122, for Doppler rotation. Data istherefore transferred out of 11 sample deep buffer 120 when 11 samplesare received, that is, at a chip rate of {fraction (1/11)}th of 2f₀ orapproximately 0.18f₀. 11 sample deep buffer 120 operates as a serial toparallel converter while parallel block 122 operates as a parallel toserial converter. This results in 186 parallel transfers per msec.

[0111] Data is shifted out of parallel block 122 at 24f₀ to 12 channelDoppler Block 108 so that the Least Significant Bit or LSB of the serialconverter, parallel block 122, is the output of CACAPT 104 in the formof CapIOut and CapQOut which are applied as CACAPT Data output 123 to 12channel Doppler Block 108. The increase in chip rate from 2f₀ to 24f₀provides an operating speed magnification of 12 as will be describedbelow in greater detail.

[0112] Referring now also to FIG. 6, 12 channel Doppler Block 108 is nowdescribed in greater detail. Doppler Block 108 receives satellitespecific CACAPT Data output 123 including CapIOut and CapQOut fromCACAPT 104 for storage in Doppler Register 124. Satellite or sourcespecific predicted Doppler phase, after processing by Carrier NumericalControl Oscillator or NCO 125 and sine/cosine look-up table 134, is alsoapplied to Doppler Register 124 where it is added to CapIOut and CapQOutfor the same SV (or other source) to form dopIOut and dopQOut. WithinDoppler Block 108, Carrier_NCO 125 operates at an effective rate of 2f₀for each satellite channel because the data sample rate is 2f₀.

[0113] For each SV, CPU 101 stores the satellite specific predictedcarrier phase dopPhaseParam, and predicted carrier frequencydopFreqParam, in RAM R2 105. Sat_Mem 186 then transfers thedopPhaseParam and dopFreqParam as shown in FIG. 9 to Carrier PhaseRegister 126 and Carrier Phase Output Buffer 128, respectively, at each1 msec boundary. In the drawings, the number of the first and last bitof the signal is provided in parenthesis, separated by a full colon, inaccordance with current conventions. Therefore, dopFreqParam is a 24 bitdigital value, the MSB of which is bit number 23 and the LSB of which isbit number 0. Adder 130 adds carrier phase to carrier frequency, derivedfrom dopPhaseParam and dopFreqParam, to produce the current carrierphase value in Carrier Phase Register 126 shown as Carrier_NCO.

[0114] The four Most Significant Bits or MSB's of Carrier_NCO in CarrierPhase Register 126 are applied to sine/cosine look-up table 134 whichincludes 2 4-bit registers for storing its output. The output ofsine/cosine look-up table 134 is applied to Doppler Multiplier 132 inDoppler Register 124 for Doppler rotation of CACAPT Data output 123(CapIOut and CapQOut) to produce rotated SV output signals dopIOut anddopQout. Doppler Register 124 uses Doppler Multiplier 132, as well asfour 4-bit registers, two adders, another pair of 5-bit registers and aquantizer to form dopIOut and dopQOut. Referring for a moment to FIG.11, dopIOut and dopQOut are applied to parallel converter 166 androtated SV output signal 127 is the output of serial to parallelconverter 166 which is applied directly to 11 bit Holding Register 140.

[0115] During each segment of time, the beginning value for the Dopplerphase of each SV is stored in RAM R2 105, retrieved therefrom by DopplerBlock 108 for the rotation of the SV during that segment. At the end ofeach segment, the end value of Doppler phase is stored in RAM R2 105 foruse as the beginning value for the next segment. Under the control ofgpsCtl 182, Doppler phase value dopP_Next in Carrier Phase Output Buffer128, saved at the end of each rotation for a particular SV by dopSave,is applied to Sat_Mem 186 for storage in RAM R2 105 for that SV, to beretrieved by Doppler Block 108 again during the next Doppler rotation ofthat SV in the following segment. The operation of Multiplexer Block 129may be best understood from the description of the triple multiplexingof ASIC 102 associated with FIGS. 10 and 11.

[0116] Referring now also to FIG. 7, 12 channel Coder Block 112 includesCoder_NCO 136 and Code Generator 138. Coder_NCO 136, which is similar toCarrier_NCO 125 shown in FIG. 6, creates Gen_Enable whenever PhaseAccumulator 148 overflows. Gen_Enable is the MSB of the output of PhaseAccumulator 148 and is applied to Code Generator 138.

[0117] In particular, under the control of gpsCtl 182, Sat_Mem 186applies the satellite specific 24 bit code frequency parameter,coderFreqParam, and the 24 bit satellite specific code phase parameter,codePhaseParam, at each 1 msec edge to Coder_NCO 136 from RAM R2 105.CoderFreqParam is added to codePhaseParam effectively at 4f₀ per channelin Phase Adder 150 even though codePhaseParam operates at 48f₀ duringtracking and reacquisition. A pulse can be generated for Gen_Enablebetween 0 Hz and 4f₀ Hz. In order to generate Gen_Enable at 2f₀, thevalue of half the bits (23:0) of Phase Accumulator 148 must be loaded inas coderFreqParam.

[0118] The LSB of codePhaseParam represents {fraction (1/256)}th of aC/A code chip. CodePhaseParam initializes the contents of PhaseAccumulator 148. Gen_Enable is generated whenever Phase Accumulator 148overflows. Phase Accumulator 148 is a 25 bit register initialized by thevalue of codePhaseParam when corHoldRegLoad 152 from CPU 101 is activeat each 1 msec edge when new data is written from CPU 101. The 24 LSB'sof 25-bit Phase Accumulator 148 are then added to coderFreqParam inPhase Adder 150 and returned to Phase Accumulator 148. Phase BufferRegister 154 stores and buffers the contents of Phase Accumulator 148,to produce CoderPNext which is updated whenever codCodeSAve 158 fromgpsCtl 182 is active. CoderPNext is applied to Sat_Mem 186 for storagein RAM R2 105. The operation of multiplexer 142 may be best understoodfrom the description below of the triple multiplexing of ASIC 102provided with FIGS. 10 and 11.

[0119] Gen_Enable is applied to Code Generator 138 to cause a new codeto be generated. C/A Codes parameters G1 and G2 are parallel loaded fromRAM R2 105 by Sat_Mem 186 as g1ParIn and g2ParIn into Code Generator 138to produce g1GenOut and g2GenOut which are returned to RAM R2 105 bySat_Mem 186. The bit-0 of both G1 and G2 generators in Code Generator138 are internally XORed and generate genSerOut 160 which is seriallyapplied to 11 bit Code Shift Register 170 in Correlator Block 110, asshown in FIG. 5. Code Generator 138 generates the following C/A codes:

[0120] G1=1+X3+X10

[0121] G2=1+X2+X3+X6+X8+X9+X10.

[0122] The output of Code Shift Register 170 is applied to correlators74, 11 bits at a time at 48f₀ so that at least 20 code delays, separatedby one half chip width, are correlated against each Doppler rotatedsample from each SV. The increase in chip rate from 2f₀ to 48f₀ providesa magnification factor of 24 as will be described below in greaterdetail.

[0123] Values of G1 and G2 are be stored in RAM R2 105 during eachsegment after correlation with the doppler rotated sample in correlators74 for that SV so that they may then be retrieved by Coder Block 112during the next time segment for correlation of the next 11 bit samplefrom the same SV.

[0124] Referring now also to FIG. 8, Correlator Block 110 is shown ingreater detail. DopIOut and dopQOut in the rotated SV output fromDoppler Block 108 are applied to serial to parallel converter 166 whichis then parallel loaded to Holding Register 140. GenSerOut 160 fromCoder Block 112 is applied to Code Shift Register 170 in CorrelatorBlock 110. These data sets represent the doppler shifted data receivedfrom the SV, as well as the locally generated code for that SV, and areapplied to Exclusive NOR gate correlator 74 for correlation undercontrol of gpsCtl 182.

[0125] The output of correlator 74 is applied to Adder 174 and combinedin Bit Combiner 176 to corIOut 178 and corQOut 180 which are applied toIQACCUM Block 114 and IQSQACCUM 116 shown in FIG. 5. Adder 174 and BitCombiner 176 operate as a partial accumulator as indicated byaccumulator 175 in FIG. 5.

[0126] Referring now also to FIG. 9, on overview of the operation ofASIC 102 is shown. A dedicated set of on-chip logic controls theoperation of ASIC 102 and is identified herein as gpsCtl 182. Inparticular, under the control of gpsCtl 182, sample data 100 from theGPS satellites is applied to CACAPT 104 where it is separated anddecimated into I and Q data streams to form CACAPT Data output 123. SVdata 123 is rotated for the predicted Doppler shift of each SV toproduce rotated SV output signals dopIOut and dopQOut which arecorrelated with genSerOut 160 from Coder Block 112 in correlators 74.CorIOut 178 and corQOut 180 from correlators 74 are accumulated inIQACCUM Block 114 and IQSQACCUM 116 to produce output 184 to CPU 101.

[0127] As will be further described below in greater detail, a portionof memory is used for Sat_Mem 186 which stores and provides the dopplershift and code information required during multiplexing.

[0128] In operation, every millisecond is divided into 186 segments,each of which includes 264 clocks. Within these 264 clocks, 12 channelsare processed with each channel taking 22 clocks to compute 22 differentcorrelations or delays. Only 20 of these 22 correlations are stored andused for subsequent processing. For each channel, gpsCtl 182 controlsthe loading of Carrier_NCO 125 in Doppler Block 108 using dopLoad anddopSave. Similarly, gpsCtl 182 controls the loading of Coder_NCO 136 inCoder Block 112 via corHoldRegLoad and corCodeSave. The flow of datathrough Correlator Block 110 is controlled with serialShiftClk, and alsocorHoldRegLoad and codCodeSave. Control signals are applied to IQACCUMBlock 114 and IQSQACCUM 116 for each channel and include startSegment,startChan, resetAcc, peak, iqsq, wrchan, ShiftSelIqSq and acq_mode.Within each segment, gpsCtl 182 provides the periodic signalseng_capShiftClk, capLoad, syncpulse, serialShiftClk to CACAPT 104 torepackage incoming satellite data samples into groups of 11 half chipsamples.

[0129] All accesses initiated by gpsCtl 182 are processed by Sat_Mem 186to generate read/write control and address signals for RAM R1 103 andRAM R2 105. GpsCtl 182 controls the flow of data through all data pathstogether with Sat_Mem 186 and manages the access of channel parametersstored in RAM R1 103 and RAM R2 105. RAM R1 103 is written to by theuser to define the channel parameters that will be loaded to RAM R2 105at the end of the corresponding integration or accumulation time. RAM R2105 is used by the datapath as a scratchpad to store the intermediatevalues of the various channel parameters during processing.

[0130] Data read out of RAM R2 105 is sent to the various parameterregisters in Doppler Block 108, Coder Block 112, Correlator Block 110and gpsCtl 182 under the control of Sat_Mem 186. Data from these blocksand RAM1 190 are multiplexed at the input to the write port of RAM R2105. RAM R1 103 is a 16×108 asynchronous dual port ram used for theparameters for all 12 channels while RAM2 192 is another 16×108asynchronous dual port ram used for storing intermediate values of thesatellite parameters during processing, while switching from one channelto the next.

[0131] Referring now to FIG. 10, the system of the present inventionincludes a multiplexed data path in order to reduce the size andcomplexity of ASIC 102 on which the majority of the parts of the systemcan be provided. Conventional receiver designs have multiplexed a singleset of correlators for use for each of the separate channels in which anSV is tracked in order to reduce the number of correlators required. Theuse of the system of the present invention reduces the million or moregates that would be required for a conventional configuration down to amanageable number, on the order of about less than 100,000.

[0132] In accordance with the present invention, in addition tomultiplexing the satellite channels in a manner in which no data islost, the code delay correlations are also multiplexed. That is,conventional receivers use two or three correlators to provide early,late and/or prompt correlations for each SV. The present inventionmultiplexes a plurality of code delays in order to provide far more codedelay correlations than have been available in conventional systemswithout substantially multiplying the hardware, or chip area on ASIC 102required by the number of gates used.

[0133] The multiplexing of code delays permits the wide capture windowdescribed above with regard to FIGS. 3 and 4 that permits rapid SVreacquisition. In particular, 20 delays such as ½ chip delays areprovided and constantly monitored for each SV so that GPS data can beacquired even during brief glimpses of the SV, for example, when car 10is in intersection 22 as shown in FIG. 1. The SV can be reacquired anduseful data obtained because the modelling of the vehicle's position onroadway 12 is sufficiently accurate to keep the predicted code anddoppler values for a previous acquired and currently obscured SV withina window of ±10 half chip code delays. In this way, data obtained duringreacquisition can be used directly as GPS data. That is, thereacquisition mode is transparent to the tracking mode. The GPS data isacquired whenever available without substantial lost time forreacquisition.

[0134] Further, the operation of satellite tracking is itselfmultiplexed for each set of data for all 12 channels in order to furthersubstantially reduce the ASIC gate count. That is, only a small portionof the bits in the C/A code is processed at one time for all 12 SV's. Inorder to digitally process the signals received, the digitalrepresentations of these signals must be processed in registers andbuffers capable of storing the digital data. The C/A code contains 1023bits in each repetition which lasts 1 msec. If all 1023 bits were to beprocessed at once, registers 1023 bits wide would be required. Suchregisters would be expensive in cost and gate count and quitecumbersome. In accordance with the third level of multiplexing used inthe triply multiplexed receiver configuration of the present invention,a smaller register is multiplexed to handle different portions of the1023 bits of the C/A code. This means the smaller register is used manytimes during each 1 msec repetition of the C/A code to process enoughsmaller samples of the data received so that within each msec all 1023bits can be processed.

[0135] In the preferred embodiment described above particularly in FIGS.3 to 9, a configuration using 11 bits registers was used so that eachregister is used 186 times per msec to process all 1023 bits of a C/Acode repetition. Each {fraction (1/186)}th of a msec is called asegment. The tracking of each SV is therefore multiplexed 186 times byprocessing the 11 bits in each register during each segment. Inaddition, in the preferred embodiment, 12 channels are used to track amaximum of 12 SV's. This requires that each 11 bit segment ismultiplexed 12 times during that segment to apply a doppler rotation foreach SV.

[0136] Further, each channel is further multiplexed by a factor of 22 toprovide a substantial plurality of different code delays. This requiresthat the doppler rotated sample for each SV is correlated 22 times withdifferent C/A Code delays before the doppler rotated sample for the nextchannel is produced. In this manner, 22 different code phases may betested for each of 12 SV during each of 186 segments to provide realtime data with only 11 bit wide registers by processing each register186 times per msec.

[0137] It is important to note that the processing of the presentinvention occurs during a particular segment, i.e. a {fraction(1/186)}th of a repetition of the C/A code, during the length of timerequired for the segment to be collected. In this optimized manner, nodata is lost during tracking or reacquisition or switching between thesestates because the data being processed in any particular segment is atmost 11 half chips delays old.

[0138] Referring now to FIGS. 10 and 11, the output of Digital Filter118 shown in FIG. 5 is sample data stream 119 at 2f₀. The chip rate ofthe C/A modulation of the signals 100 from the SVs is at f₀. In order toavoid loss of any data, the SV signals must be sampled at least at theirNyquist rate, that is, at twice the chip rate of the modulation ofinterest which is 2f₀. Although sample data stream 119 can be operatedat a higher chip rate than the Nyquist rate, which is twice the chiprate, there is no advantage in doing so.

[0139] Sample data stream 119 is therefore a series of samples of thedigitized and filtered SV data at twice the chip rate of the C/A code,that is, each sample in sample data stream 119 has a width equal to onehalf of a C/A code chip. The number of bits in each msec or cycle ofcode in sample data stream 119 is twice the number of bits in themodulation, i.e. 2046 bits each representing one half of a C/A codechip. In accordance with the multiplexing scheme of the preferredembodiment being disclosed, the data is processed in 11 bit segments,and sample data stream 119 is therefore applied serially to 11 bit(10:0) register value buffer 120. The time required to serially store 11bits out of a total of 2046 bits in the 2f₀ data stream is1÷(2046÷11=186) or {fraction (1/186)}th of a msec.

[0140] During the time the first set of 11 sample bits are being storedin 11 sample deep buffer 120, no bits are available for processing.After the first 11 sample bits are serially received and seriallystored, the 11 sample bits are transferred in parallel to parallel block122. This parallel operation therefore occurs every {fraction (1/186)}thof a msec or at a rate of approximately 0.18f₀. Each {fraction(1/186)}th of a msec is called a time segment or segment and is the unitof processing for most of the operations. The 1023 chip C/A code of eachof the satellites in the composite signal received is processed in 11half chip bits. Dividing the msec repetition rate of the C/A code into186 time segments multiplexes each of the 11 bit registers by amultiplexing factor of 186.

[0141] CACAPT Data output 123 from parallel, block 122 is processed inDoppler Block 108 at a much faster chip rate, for example at 24f₀. Thatis, the 11 bits of sample data in each segment of time is multiplexed bya factor of 12 to permit 12 different operations to be performed to thatset of 11 bits of data. In particular, in Doppler Block 108, CapIOut andCapQOut of CACAPT Data output 123 are multiplied in Doppler Register 124by twelve different doppler shifts so that within each segment twelvedifferent doppler rotations are performed.

[0142] Each different doppler shift represents the predicted dopplerrotation required for each of the maximum of 12 different Svs that canbe tracked. The increase in processing chip rate from 2f₀ to 24f₀multiplexes the processing for each of 12 channels of data. It isimportant to note that the multiplexing to permit one channel to operateas 12 multiplexed or virtual channels each representing a different SVis applied only after the input signals are multiplexed, that is, brokeninto 186 time segments each including 11 half chip width bits. In thisway, the multiplexing for 12 channels or satellites is easilyaccomplished with relatively inexpensive 11 bit registers without lossof time or data. The selection of the number of sampling to be aninteger division of the number of code bits per period is important toachieve these goals. Multiplexer Block 129 in Carrier_NCO 125 controlsthe timing of this multiplexing under the direction of gpsCtl 182.

[0143] The output of Doppler Block 108, signals dopIOut and dopQOut, areapplied to serial to parallel converter 166 within Correlator Block 110.Each rotated SV output signal 127 represents the rotated signal from asingle SV and 12 such rotated SV output signals 127 are produced in eachsegment of time.

[0144] Rotated SV output signal 127 is loaded in parallel fashion intoHolding Register 140 in Correlator Block 110. The input to Exclusive NORgate correlator 74 is therefore an 11 bit wide signal which is retainedfor {fraction (1/12)}th of a time segment as one input to Exclusive NORgate correlator 74.

[0145] Correlator 74 is a series of 11 separate one bit correlatorswhich all operate in parallel. One input is rotated SV output signal 127while the other 11 bit input is provided by 11 one bit genSerOut 160output bits from Coder Block 112. During the {fraction (1/12)} of a timesegment provided for operation on the rotated SV output signal 127 for aparticular satellite, the code for that SV is produced serially by CodeGenerator 138 and applied to Code Shift Register 170.

[0146] At the beginning of the correlation for a particular channel, 11bits of the code for that SV have been shifted into Code Shift Register170 and are available therein for correlation. Every {fraction (1/22)}ndof a channel (that is, a {fraction (1/12)} of a segment) each of the 11bits in Code Shift Register 170 are correlated in one of 11 one bitexclusive Nor gates in Exclusive NOR gate correlator 74. This produces11 correlator output bits, the sum of which indicates the magnitude ofthe correlation between the rotated SV output signal 127 and that codephase. These 11 correlation sums produced in parallel are summed inparallel and stored in the first of 22 summers related to that SV inAccumulator Block 115.

[0147] During the next or second {fraction (1/22)}nd of a channel, CodeGenerator 138 produces the next bit for the C/A code for that SV. Thisnext bit is applied serially to Code Shift Register 170. At this time,10 bits from the first correlation remain in Code Shift Register 170 andtogether with the newest bit form another 11 bit sample of the expectedcode for that SV, delayed from the previous 11 bit sample by the timerequired to generate 1 bit, that is, one half chip width at the ratecode is produced, 48f₀ The second sample is therefore a one half chipdelayed version of the code, delayed one half chip width from theprevious 11 bit samples. It is important to note that the two 11 bitcode samples just described differ only in that a new bit was shifted inat one end of the register to shift out the MSB at the other end of theregister.

[0148] The 11 bit correlation product of the same rotated SV outputsignal 127 and the second 11 bit sample of code is then stored in thesecond of the 22 summers related to that SV in Accumulator Block 115.Thereafter, the remaining 20 serial shifts of the genSerOut 160 fromCode Generator 138 are correlated against the same rotated SV outputsignal 127 to produce 20 more sums of 11 bit correlations for storage inAccumulator Block 115 for that SV. The result is that 22 values are thenavailable within Accumulator Block 115 for processing, each value is ameasure of the correlation of the signals from one SV with 22 differentcode phases or delays, each separated by one half chip width.

[0149] During the next {fraction (1/12)} of a time segment, that is,during the processing of the second multiplexed channel, the rotated SVoutput signal 127 for the next SV, is applied to Holding Register 140for correlation with 22 different one half chip delays of the codegenerated for that satellite. At the end of a segment, Accumulator Block115 includes a matrix of 12 by 20 different sums. In one implementationof the present invention, it has been found to be convenient to saveonly 20 out of the 22 possible code delay correlation results. The 12rows of 20 sums represents the measure of correlation for each of the 12SV's at 20 code phases or delays.

[0150] In summary, the data path for the present invention is triplymultiplexed in that

[0151] (a) each msec, which represents 1023 bits of C/A code, is slicedinto 186 to form the 186 segments in a msec of sample so that only 11half chip wide sample bits are processed at one time;

[0152] (b) each segment is then multiplexed by 12 so that each such 11bit sample is rotated for twelve different sources;

[0153] (c) the rotated 11 bit sample for each source is correlatedagainst 20 sets of different code delays for that source to multiplexwithin each channel by 20; and

[0154] (d) the sum of the correlation products for each delay in eachchannel are then summed to produce the accumulated correlation output.

[0155] Although 22 different delays are available, it is convenient touse 20 such delays, or code phase theories for testing the rotatedsatellite signal. The correlation product having the greatest magnitudefor each channel after accumulation, that is, the largest of the 20 sumsof 11 bits stored in Accumulator Block 115 for each channel may then bedetected by its magnitude, for example by a peak detector, to determinewhich delay theory is the most accurate. The peak sum represents theon-time or prompt correlation for that SV.

[0156] Turning now specifically to FIG. 11, the triple multiplexingscheme of the present invention may easily be understood by looking atthe slices of time resulting from each of the multiplexing operations.Within each msec, the C/A code for each particular satellite has 1023bits. In order to preserve all necessary information, the satellitesignals are sampled, in a digital composite of signals from allsatellites, at the Nyquist rate at 2f₀ to produce 2046 half chip widesample bits.

[0157] Each sequential set of eleven sample bits are processed togetheras a segment of time, the length of which is equal to 1/(2046÷11) of amsec, i.e. one {fraction (1/186)}th of a msec. After processing of the186th segment in a msec all necessary data has been extracted and the 11bit sample for the next segment is available. Although the partial sumsaccumulated over each msec in Accumulator Block 115 may only beevaluated at the end of a msec, no data is lost and the results are only1 segment late. That is, since it takes 1 segment to fill 11 sample deepbuffer 120 and transfer the 11 bit sample to parallel block 122, thedata from the first 11 bit sample is being processed while the data forthe second 11 bit sample is being collected. Even if the system operatedfor a year, the sampled being processed to provide position informationis still only one time segment old.

[0158] The 11 bits of each segment are multiplexed for each SV by beingtime division multiplexed during doppler rotation. That is, the 11 bitsample of segment 1 is used to provide 12 different doppler shiftedoutputs so that a single 11 bit segment sample is used 12 times toproduce 12 different satellite specific doppler rotated versions,assuming all 12 satellites are in view or being modeled. The operationsfor one channel then require one twelfth of a segment. It is critical tonote that each segment only produces a partial result and that the 12partial results during each segment must be summed at the end of eachmsec to provide valid output data.

[0159] Each of the operations on one particular channel in a segment aretime division multiplexed by a factor of 22 so that 22 different codedelays for that partial sum for that satellite can be tested. The peaksum of these 22 correlations can however be detected by magnitudeimmediately if necessary to select the most likely delay for thatchannel. In the present embodiment, the information for that channel isonly valid once per msec when summed or accumulated so that there maynot be a substantial advantage in peak detected with a particularsegment. In some GPS applications and in other spread spectrumapplications, such as wireless communications, it may be desirable ifstrong signals are present to accumulate and transfer the sum of theaccumulations for each source from R3 to R4 more often than once percode repetition rate. The time required to evaluate a particular codephase delay or theory is only {fraction (1/22)}nd of the time requiredper channel per segment or {fraction (1/22)}nd of {fraction (1/12)} of{fraction (1/186)}th of a msec. This speed of operation is more easilyachieved because the 11 one bit correlations required are produced inparallel. Similarly, the speed of generation of the different codedelays for a particular SV is more easily accomplished in accordancewith the present invention because each 11 bit code delay sample isautomatically produced when each single new bit, i.e. each new genSerOut160, is shifted into Code Shift Register 170.

[0160] The selection of the magnitudes or multiplexing factors used ineach level of multiplexing is not arbitrary. The larger the number ofsegments, the smaller the required size or depth of the registers needfor each sample. By using a code repetition multiplexing factor of 186,that is, by dividing the 2046 bits of a 2f₀ by 186, only 11 sample bit,need to be evaluated at a time.

[0161] The number of required channels is bounded pragmatically by thefact that at least 4 Svs must be in view at the same time to determineposition accurately in three dimensions. Time is the fourth unknownwhich must be determined along with each of the three dimensionsalthough provisions for estimating, modeling and/or updating theposition information as described above so that position information maybe accurately provided even during periods when less than 4 satellitesare concurrently in view.

[0162] The constellation of 24 NAVSTAR satellites in use are arranged tocover the earth so that a maximum of 12 such satellites may be in viewat any one location at any particular time. The maximum number ofpragmatically useful channels is, for this reason, no less than about 12channels. The selected channel multiplexing factor used in the channellevel of multiplexing in the embodiment shown herein is therefore afactor of 12.

[0163] The number of different code delays is bounded at the low end byan absolute minimum of 1 so that if the exact delay can somehow bemaintained, the only necessary correlation would be the on-time orprompt correlation. Conventional GPS receiver systems use at least 2 or3 different code delays so that conventional tracking techniques, forexample those which use early, prompt and late correlations to centerthe prompt correlation within ±1 delay, may be employed.

[0164] In accordance with the present invention, a substantially greaternumber of different code delays, or delay theories, are tested so thatfast reacquisition may be accomplished as described above with regard toFIGS. 3 and 4. Although for the particular preferred embodimentdescribed herein, it was determined that a total of 20 different delays,each separated in time by one half the width of a C/A code chip, i.e. ½of {fraction (1/2046)} of one msec, a code delay multiplexing factor of22 was selected because the relationship between each of the 3multiplexing factors is also important.

[0165] The product of the three multiplexing factors, code repetitionmultiplexing factor, channel multiplexing factor and code delaymultiplexing factor should optimally be an even integer multiple of thenumber of bits in each repetition of the spread spectrum modulation. Aneven integer multiple is required because samples must be taken at twicethe chip rate, i.e. at the Nyquist rate, in order to avoid data lossfrom sampling at a slower rate. Although multiplexing factors can beused successfully even if the product is not exactly equal to an eveninteger multiple, data loss or unnecessary complexity and costs mayresult.

[0166] In the particular embodiment shown, the spread spectrum code ofinterest is the C/A code, each repetition of which includes 1023 bits.In accordance with the triple multiplexing product rule discussed above,the product of the three multiplexing factors must equal an even integermultiple of 1023, such as 2046. In the described embodiment, the coderepetition multiplexing factor is 186, the channel multiplexing factoris 12 and the code delay multiplexing factor is 22. The product of 186multiplied by 12 and then by 22 is 49104 which, when divided by 1023,equals 48. 48 is an even integer and therefore the particular set ofmultiplexing factors used in the present invention provides one ofseveral optimized systems.

[0167] The reason this multiplexing factor product rule works well in atrilevel multiplexing configuration for C/A code is that there are threeprime factors in 1023. That is, 1023 is the product of three primenumbers, 31, 11 and 3. Each of the three multiplexing factors is evenlydivisible by one of these prime numbers. For example, 186 is divisibleby 31 six times, 12 is divisible by 3 four times and 22 is divisible by11 twice.

[0168] Using each prime factor of the number of bits in the sampled bitrate in one of the multiplexing factors yields two or more differentfamilies of multiplexing configurations for C/A code spread spectrumreceivers. In the first family, if 11 channels are desired, then eitherthe code repetition multiplexing factor or the channel multiplexingfactor would have to be divisible by 31. Although it may be desirable incertain applications to use 31 or 62 different code delays, there is asubstantial advantage in making the code repetition multiplexing factoras large as possible. This reduces the number of bits required to besaved and processed in each segment. By selecting the code repetitionmultiplexing factor to be a multiple of 31, the number of delaysactually used can be more easily controlled because the code delaymultiplexing factor could be any multiple of 3.

[0169] In the other convenient family, 6, 9, 12, 15 or 18 satellitechannels are desired so that the channel multiplexing factor is anintegral multiple of 3. This permits the code delay multiplexing factorto be a factor of 11 while the code repetition multiplexing factor is afactor of 31. The particular embodiment described in the specificationabove is in this family.

[0170] Another constraint on the selection of multiplexing factors isthe speed of operation of the lowest level of multiplexing. In theembodiment disclosed, the third level of multiplexing operates at 48f₀.The clock speed of the hardware implementation must be sufficient topermit operation at this speed. As faster and faster on chip componentsare developed, higher clock speeds may be used to accomplish the highestspeed processing and larger multiples may be used. For example, withcomponents in the high speed processing sections such as CorrelatorBlock 110 capable of operation at higher rates at multiples of f₀, suchas at 96f₀, the code repetition multiplexing factor could be doubled toproduce 24 channels with 20 delays or taps or 12 channels with 40 delaysor taps or 11 channels with 6 bits and 22 taps.

[0171] The system configuration may also be viewed from the standpointof a time or speed magnification. Operation at the third multiplexinglevel at 48f₀ is 24 times faster than the chip rate of the 2f₀ samplebeing processed. This amplification factor of 24 permits a hardwaremultiplexing or gate compression factor of 24. The number of gates onASIC 102, or other devices for implementation the present invention, isreduced essentially in direct proportion to the magnification factor.All other factors being equal, the surface area of a chip operated at48f₀ is on the order of {fraction (1/24)}th of the surface area thatwould be required to operate at 2f₀. Similarly, an increase in themagnification factor to 96 would permit a reduction in the required chipsurface real estate required on the order of almost half.

[0172] The particular embodiment of the multiple level multiplexingspread spectrum receiver of the present invention which has beendisclosed above is a GPS receiver. The same invention can be used forother spread spectrum signals such as wireless telephone signals withdue consideration for the selections of multiplexing factors based onthe bit rate of the spread spectrum code used and the environmentalfactors applicable to that application. The environmental factors forthe present configuration, such as the pragmatic constraints on thenumber of channels and code phases, have been described above.

[0173] Having now described the invention in accordance with therequirements of the patent statutes, those skilled in this art willunderstand how to make changes and modifications in the presentinvention to meet their specific requirements or conditions. Suchchanges and modifications may be made without departing from the scopeand spirit of the invention as set forth in the following claims.

What is claimed is:
 1. A method of receiving signals modulated by aspectrum spreading code having a fixed number of bits repeated during afixed length time period being transmitted from a plurality oftransmitters, comprising: dividing the fixed length time period into anumber of time segments evenly divisible into twice the fixed number ofbits; applying each time segment to each of a number of channels, eachof said channels being used for tracking one of said plurality oftransmitters; and applying each of said time segments in each of saidchannels to a number of code phase delay tests.
 2. The method of claim 1wherein applying each of said time segments in each of said channels toa number of code phase delay tests comprises: correlating the bits ineach time segment in each channel in parallel with a source specificseries of locally generated sequentially delayed code samples; summingeach parallel correlation; and accumulating the summed parallelcorrelations for each code sample in each channel.
 3. The method ofclaim 2 wherein the series of delayed code samples differ by one bit. 4.The method of claim 2 wherein the summed parallel correlations areaccumulated at a rate at least equal to the chip rate to derive datarelated to each of the sources.
 5. The method of claim 1 wherein thenumber of time segments is an integral multiple of
 31. 6. The method ofclaim 5 wherein the number of channels is an integral multiple of 3 andthe number of code phase delay tests is an integral multiple of
 11. 7.The method of claim 5 wherein the number of channels is an integralmultiple of 11 and the number of code phase delay tests is an integralmultiple of
 3. 8. The method of claim 1 wherein the number of channelsis an integral multiple of
 3. 9. The invention of claim 1 wherein thenumber of code phase delay tests is an integral multiple of
 11. 10. Themethod of claim 11 wherein the product of the number of time segments,the number of channels, and the number of code phase delay tests is aneven integer multiple of the number of bits.
 11. A method of navigatingcomprising: processing signals from a plurality of transmitting sourcesmodulated by different spread spectrum codes, each having a fixed numberof bits repeated during a fixed length time period, to provide datarelated to the position of an object, the processing step comprising:dividing the fixed length time period into a number of time segmentsevenly divisible into the twice the fixed number of bits; applying eachtime segment to each of a number of channels, each of said channelsbeing used for tracking one of said plurality of transmitters; applyingeach of said time segments in each of said channels to a number of codephase delay tests; and processing the derived data to determine theposition-related data; in response to the position-related data,determining a navigation solution; and in response to the navigationsolution, providing navigation information related to the object. 12.The method of claim 11 further comprising: providing information relatedto the local position of the object; and during the step of determininga navigation solution, processing the local-position information withthe position-related data to determine the navigation solution.
 13. Themethod of claim 11 further comprising: obtaining data related to thephysical environment through which the object is being navigated; andduring the step of determining a navigation solution, processing thephysical environment data with the position-related data to determinethe navigation solution.
 14. The method of claim 13 wherein thephysical-environment data is obtained from a map data abase.
 15. Themethod of claim 11 further comprising: providing information related tothe local position of the object; obtaining data related to the physicalenvironment through which the object is being navigated; and during thestep of determining a navigation solution, processing the local-positioninformation and the physical-environment data with the position-relateddata to determine the navigation solution.
 16. The method of claim 11wherein the position-related data comprises at least one of surfaceelevation, satellite time, two-dimensional orthogonal components of theearth surface and a pair of orthogonal directions related to thedirection of object movement.
 17. The method of claim 11 wherein thenavigation information comprises at least one of visual information andaudio information.